The present invention relates generally to image forming equipment and is particularly directed to color laser printers of the type which have transfer belts that receive latent images from multiple photoconductive members. The invention is specifically disclosed as a motion control system that maintains a substantially constant belt velocity under varying environmental conditions and for various styles of drive motors and variations in individual belt physical parameters.
In color printers a plurality of color planes are sequentially aligned and deposited onto a transfer media such as a transfer belt. The transfer belt is then used to transfer the accumulated color planes to a piece of paper or other media. A problem associated with this process is misregistration or misalignment of one or more of the color planes. Alignment of the color planes is crucial in achieving a high quality image. Due to the fact that each individual color plane is transferred onto the belt or paper at different locations along the travel path of the transfer belt, the belt position within the travel path must be controlled with a high degree of precision. The motion of the drive motor that drives the belt must be accurately controlled to insure that there is little or no misregistration of the color planes on the belt such that the resulting image is of good quality.
There are many instances where motion inaccuracy can develop and cause a concomitant degradation in the resulting image. Factors such as variations in the thickness of the belt, variations in the belt tension, and variations in the drive motor system itself are examples of factors that lead to motion inaccuracy.
Motor control systems of color printers usually sense motor position by means of an encoder and control the motor driver such that pulses produced by the encoder coincide with clock pulses generated by the controller. This adds cost and complexity to the printer. It would be desirable to have a method and apparatus that corrects for motion inaccuracy which is inexpensive to implement and does not add complexity to the printer.
A transfer belt subassembly for a color printer includes a transfer belt, a home position indicator, a temperature sensor and a memory. The transfer belt subassembly is measured and characterized after its fabrication and before being installed in a printer. The measurement and calibration data for the transfer belt is stored in the memory that is part of the subassembly. The memory stores data representing the motion characteristics of the transfer belt, such as velocity characteristics and temperature compensation factors for use by an engine-controller (which may be defined as one or more integrated circuits, including a microprocessor or logic state machine, firmware, and memory) of the printer to govern the motion control of the drive motor. When the transfer belt subassembly is inserted into a printer, the engine-controller in the printer is placed in communication with the memory. Sensors are employed to determine the home position of the transfer belt and to provide a measure of belt velocity and temperature. The engine-controller utilizes the characterizing data from the memory and temperature sensor data (such as the output of a thermistor) to provide adjustment of belt velocity and compensation for variations in the transfer belt motion quality. By use of the predetermined characterizing data, precise alignment of the color planes with respect to one another is achieved for accurate color printing.
In one embodiment, two belt sensors are used for velocity control of the belt. In another embodiment, only a single belt sensor is used for belt velocity control. In both preferred embodiments, a temperature sensor is used to correct for temperature variations that can affect the physical characteristics of the belt.
It is an advantage of the present invention to provide a motion control system that controls the velocity of a moving belt member of an electrophotographic printer, while correcting for variations in environmental conditions or variations in individual belt parameters.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, an apparatus for providing transfer quality optimization of color planes transferred to or from a transfer belt of an image forming apparatus is provided, which comprises: a plurality of transfer rollers; a transfer belt disposed about the plurality of transfer rollers; a memory capable of storing data relating to the transfer belt at multiple transfer stations; a home position indicator associated with the transfer belt; first and second sensors for sensing the home position indicator; and a temperature sensor for sensing temperature near a surface of the transfer belt.
In accordance with another aspect of the present invention, an apparatus for providing transfer belt position correction, used in a color printer having a plurality of color planes deposited onto a transfer belt, comprises: a transfer belt subassembly including: (a) a transfer belt disposed about a plurality of rollers and having a home position indicator; (b) a temperature sensor disposed to sense temperature near a surface of the transfer belt and to provide a signal representative thereof; and (c) a memory capable of storing transfer belt calibration data.
In accordance with a further aspect of the present invention, a method of controlling transfer belt position in a color printer is provided, in which the color printer has a plurality of color stations, a transfer belt subassembly having a transfer belt disposed about a plurality of rollers, a temperature sensor, a belt position sensor, a memory, and a variable speed motor for driving the transfer belt about the rollers, the method comprising: storing characterizing data for the transfer belt in the memory which represents the measured velocity profile for the transfer belt; and providing drive signals to the variable speed motor in response to data from the memory and signals from the sensors to control the speed of the motor and the speed of the transfer belt to provide nearly constant surface velocity between color stations of the printer.
In accordance with still a further aspect of the present invention, a printer having a motion-controlled transfer belt is provided, comprising: a plurality of rollers; a transfer belt disposed about the plurality of rollers; an indicator disposed on the transfer belt; a plurality of sensors disposed adjacent the transfer belt, each of the plurality of sensors capable of sensing the indicator; a memory for storing data representing transfer belt characteristics; a motor for driving the transfer belt; and a controller in communication with the plurality of sensors, the memory and the motor, the controller operative to adjust the speed of the motor in accordance with the contents of the memory to compensate for motion inaccuracy of the transfer belt based on the velocity profile of the transfer belt.
In accordance with yet another aspect of the present invention, an image forming apparatus having a motion-controlled transfer belt is provided, comprising: a plurality of rollers; a transfer belt disposed about the plurality of rollers; an indicator disposed on the transfer belt; a sensor disposed adjacent the transfer belt, for sensing the indicator; a memory for storing data representing transfer belt characteristics; a motor for driving the transfer belt; a controller in communication with the sensor, the memory, and the motor, the controller operative to run the transfer belt at a predetermined default motor speed for an entire belt revolution, as detected by the position sensor; and the controller being further operative to count motor output pulses during the belt revolution, and to adjust the belt speed accordingly to run at a substantially constant velocity.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.